This invention relates generally to a system and method for tightening fastener assemblies, and threaded fasteners in particular. The invention is particularly suited to the tightening of threaded fasteners to the yield point of the material comprising the assembly.
There are many different classes and types of structural joints, many of which joints are secured by mechanical fasteners. In the design of such structural joints, a desired clamping force or compressive force on the two members comprising the joint is predetermined. In some cases the predetermined clamping force is within the elastic region of the fastener, while for other applications it is desirable that the maximum clamping force be exerted and that the fastener be tightened to its yield point. Of course, uncontrolled tightening the fastener beyond its yield point could result in gross permanent deformation of the fastener, and would not be desirable.
The prior art is replete with devices and wrenching systems for applying a predetermined magnitude of torque to a threaded fastener, as evidenced by the great number of conventional torque wrenches available. One problem with conventional torque wrenches has been, as has generally been accepted, an approximate .+-.30% error in the correlation between the torque applied to the fastener and its resultant axial tension. This 30% error is due to variations in the coefficient of friction between the fastener and the members being joined, which results from tolerance variations and differences in the surface condition of the mating faces of the fastener and members being joined, among other variables.
U.S. Pat. No. 3,982,419 issued Sept. 28, 1976 to Boys discloses one possible tightening system for eliminating the 30% error between torque applied and the corresponding axial tension induced in the fastener assembly. The Boys system teaches the use of a wrench and appropriate electronics to tighten a fastener assembly to its yield point by developing a signal representative of the gradient of a torque vs. rotation curve, storing a gradient signal representative of the linear tightening region, and discontinuing tightening when an instantaneous gradient signal is a predetermined percentage of the stored gradient signal. While such a system is capable of accurately tightening a fastener assembly to its yield point, it may be unduly sophisticated and expensive for use in the field where a smaller, less expensive and portable device is needed.